I just gave mine a hundred dollar bill... Lesson and gift. Giving him cash for the last few rewards/holidays has really gotten him spending his money frugally. Even been having him do price shopping on amazon etc. FTR he went to game stop with it. I also like amazon cards.. But giving cards for a place that you may have to use it to pay shipping kind of blows as a present.
"Thanks for the 25 $ shipping dad"

Also gave him an XBOX card a while ago, so he could buy a couple games and some goofy stuff for his avatar on that.